The present invention relates to imaging techniques which require magnetic field gradients, for example, nuclear magnetic resonance imaging and electron spin resonance imaging, and more particularly to the design, manufacture and fabrication of magnetic field gradient coils and assemblies.
Conventional techniques for fabricating, manufacturing and designing gradient coils have employed a single conductor as the basic "building block". The single conductor is wound continuously until desired characteristics have been attained for a given gradient coil, assembly and/or section. The gradient coils designed and fabricated using a single conductor approach tend to exhibit undesirable electrical characteristics including large impedances which result in relatively slow switching speeds. For example, the switching time is often a primary limitation to the speed of the nuclear magnetic resonance imaging applications.
The construction of gradient coils employing the single conductor construction is both tedious and time consuming due, in part, to the nature of the precision winding process. Manual construction of conventional gradient coils may require numerous hours to fully complete. Generally, great care must be taken to maintain the winding uniformity when wrapping the wire onto a coil former. Conventional gradient coils employ larger or higher gauge conductors (often copper wires) having small diameters. Larger diameters (smaller gauge) are much less pliable than are wires with smaller diameters. It is much more difficult to maintain winding uniformity when using larger diameter wires.
The winding uniformity is more easily maintained when wrapping circular coils (e.g., Maxwell coils) than when wrapping non-circular coils (e.g., saddle coils and quadrupolar coils). Circular coils are wound in a single plane, whereas saddle coils and quadrupolar coils are wound to conform to non-planar geometries.
Different approaches to an "optimal" design of magnetic field gradient coils have been described in the literature including earlier work by Golay, "Homogenizing Coils for NMR Apparatus," U.S. Pat. No. 3,622,869 as well as work by Schenck et al., "Transverse Gradient Field Coils for Nuclear Magnetic Resonance Imaging," U.S. Pat. No. 4,646,024; Wong et al., "Coil Optimization for MRI by Conjugate Gradient Descent," Magn. Reson. Med. 21 (1991) 39; Romeo et al., "Magnet Field Profiling: Analysis and Correcting Coil Design," Magn. Reson. Med. 1 (1984) 44; and Turner, "A Target Field Approach to Optimal Coil Design", J Phys. D: Appl. Phys. 19 (1986) L147; "Minimum Inductance Coils", J Phys. E: Sci. Instrum. 21 (1988) 952.
Bowtell et al. in, "Screened Coil Designs for NMR Imaging in Magnets with Transverse Field Geometry", Meas. Sci. Technol. 1 (1990) 431, describes a gradient coil design and fabrication technique involving etching of current patterns in copper sheets. The copper sheets are wrapped around a cylindrical former, resulting in a completed gradient coil assembly. While this technique is not limited to single layer structures, multi-layer designs require alignment and interconnection. These techniques require elaborate and expensive electronic equipment and facilities which tends to be cost-prohibitive for many groups working in the field.
Basically, the difficulties in constructing gradient coils and in achieving short switching times with gradient coils are somewhat inter-related. The difficulty in construction quickly leads a gradient coil designer to use small, flexible wires (often 18 gauge or smaller) which will bend more easily and conform more readily to a desired shape. The use of small wires, however, establishes a limit to the maximum current that may be safely applied to the gradient coil. As a result, to achieve a desired field gradient strength with such small diameter wires, a larger number of turns is necessary to achieve the desired value of magnetic field gradient. The larger number of turns yields a larger value of gradient coil impedance which results in a slower switching speeds of the coil.
As a result, there exists a need for an alternative approach to the design and construction of customized magnetic field gradient coils for nuclear magnetic resonance imaging. There exists a need for an inexpensive gradient coil which is easy to fabricate from inexpensive commercially available and commercially abundant materials. There exists a need for a gradient coil having a small impedance providing fast switching speeds which may be remotely and electronically switchable into various modes of operation providing desired electrical characteristics such as rapid rise times with reduced gradient magnitude or higher gradients with relatively longer rise times. Finally, there exists a need for a gradient coil permitting use of an inexpensive, lower voltage rating power supply.